The recent trend for a mobile phone becoming multi-functional and multi-banded has entailed a growing increase in the number of components engaged in forming the so-called RF front-end section thereof. However, due to the market preference toward a compact mobile phone, the size and the weight of each component are required to be further reduced.
On the other hand, many filters have been commonly used at an RF stage and an IF stage in each relevant component employed in the mobile phone to meet the requirements of low loss, excellent out-of-band attenuation characteristics and wide bandwidth. However, to meet the trendy requirement of reducing the number of components employed, new functions are required to be added to each component. For instance, an unbalanced input/balanced output or a balanced input/unbalanced output type filter device has been proposed. As used herein, the balanced input or the balanced output means that a signal is inputted or outputted as a difference in potential between two signal lines, wherein signals from the two signal lines are equal in amplitude but in reverse phase relationship; whereas the unbalanced input or the unbalanced output means that a signal is inputted or outputted as a potential of one signal line with respect to a ground potential.
Further, to meet the market demand of equipping a mobile phone with a plurality of communication systems, it has become necessary to use a plurality of filter devices having different frequency bands. Again, in response to the requirement of reduction in the number of components and the size of the mobile phone, filter devices having a plurality of filter elements arranged on a single circuit board and having the capability of multi-bands have been proposed.
Meanwhile, a duplexer is often used as one of the components placed directly under an antenna. The duplexer is a device which functions to separate signals in different frequency bands. For example, a duplexer which serves to separate the frequency bands for transmission from those for reception has been used in CDMA mobile phones.
Conventionally, a dielectric resonator filter has been used in filter devices and duplexers. However, due to the trend toward the miniaturization of devices, a surface acoustic wave filter or a piezoelectric thin-film resonator filter has been recently used in its stead. The surface acoustic wave filter, an element using surface acoustic waves excited by IDT (Inter Digital Transducer) electrodes formed on a piezoelectric substrate, is disclosed in Japanese Patent Laid-open Application No. 1993-167388. The contents of this publication are incorporated herein by reference in their entirety. On the other hand, the piezoelectric thin-film resonator filter is an element using a piezoelectric thin-film resonator provided on a Si substrate or the like and acoustically separated from the substrate. Japanese Patent Laid-open Application No. 2005-73175 discloses the piezoelectric thin-film resonator; and the contents of this publication are incorporated herein by reference in their entirety.
The duplexer is formed by connecting a plurality of filter devices having different frequency passbands. The exemplary configuration of a duplexer disclosed in Japanese Patent Laid-open Application No. 1993-167388 is reproduced in FIG. 10, wherein a first filter F50 and a second filter F60 are interconnected at a common electrode 500 to which an antenna terminal 510 is connected.
For example, in case the first filter F50 is a filter passing a transmission frequency band (hereinafter, referred to as “Tx filter”), and the second filter F60 is a filter passing a receiving frequency band (hereinafter, referred to as “Rx filter”), a received signal is sent from the antenna terminal 510 to a receiver circuit through the second filter F60 (Rx filter) and a receiving terminal 530. Further, a signal from a transmitter circuit is transmitted to the antenna terminal 510 through a transmission terminal 520 and the first filter F50 (Tx filter).
However, in the circuit configuration of FIG. 10, for example, a transmission signal may leak from the common electrode 500 to the receiver circuit side as well as to the antenna terminal 510. Therefore, there is provided a matching circuit L501, for example, an inductor, between the antenna terminal 510 and each filter. Due to the matching circuit L501, the transmitter circuit acts like a near infinite impedance to the antenna terminal 510 in the receiving frequency band whereas the receiver circuit acts like a near infinite impedance to the transmitter circuit in the transmission frequency band.
As described previously, a filter device is required to have electrical characteristics such as low loss, excellent out-of-band attenuation characteristics and wide bandwidth. Further, the filter device is generally constructed by combining a filter element with an inductor in order to improve the electrical characteristics. Furthermore, the unbalanced input/balanced output or the balanced input/unbalanced output type filter device in general requires an impedance matching, which is usually adjusted by combining the filter element with the inductor.
Moreover, although there have been proposed various methods for constructing a matching circuit for the duplexer, the matching circuit is usually formed by combining an inductor with a capacitor. In the duplexer illustrated in FIG. 10, the inductor L501 is connected between the common electrode 500 and a ground potential, thereby obtaining an effect of the impedance matching as mentioned above. The inductor L501 has an inductance ranging approximately from 3 nH to 15 nH in case the first filter has a center frequency of 836 MHz and the second filter has a center frequency of 881 MHz.
In a practical duplexer, the inductor L501 is formed of a lumped element such as a coil arranged apart from the filter element or of a strip line installed in an internal layer of a package in which the filter element is placed. This aspect has been disclosed in Japanese Patent Laid-open Application No. 2003-304139. The contents of this publication are incorporated herein by reference in their entirety. In case the lumped element is arranged apart from the filter element, a relatively high-Q inductor can be used, resulting in a low loss duplexer. However, since the lumped element has to be disposed besides the duplexer, a larger area is required for mounting them on a circuit board.
In contrast, in case the strip line is installed in the internal layer of the package, only the duplexer, not the lumped element, becomes necessary, so that the area for mounting it on the circuit board can be smaller. However, since the strip line has a relatively low-Q value, it may entail an increase in the loss. Besides, in order to obtain an inductance of approximately 10 nH as stated above, in case, for example, a material having a dielectric constant of about 9 is used in the package having a thickness of about 0.1 mm, and a line width of the strip line is about 100 μm, the strip line will be required to have a length of about 20 mm. Therefore, it has to be stretched over a plurality of layers in a multilayer package as disclosed in Japanese Patent Laid-open Application No. 2003-304139, or the package needs to be enlarged, which goes against the market trend toward the reduction of the duplexer in size and weight.
Meanwhile, in Japanese Patent Laid-open Application No. 1992-365214, a method for forming a line with an inductance component on a substrate in accordance with a conventional manufacturing process of the filter element is disclosed. The contents of this publication are incorporated herein by reference in their entirety. However, in accordance with this film forming method using a vacuum apparatus, the line should be actually formed of a thin film having the thickness of at most several μm so that a resistance component becomes large, and a Q value of the line becomes low, thereby deteriorating the filter characteristics.
To overcome the problems, the present invention provides a compact filter device including a low loss inductor, a multiband filter, a duplexer and a communications equipment, each of the latter three using the filter device.